Thermal spray coating is appropriate terminology to describe generically a group of well-known processes for depositing metallic, non-metallic or mixed non-metallic/metallic coatings. Common to thermal spray coating processes are that they require a heat source, a propelling means and a feed material to produce the coating system and also that the material to be deposited is used as is or converted to a very fine particulate state, desirably atomized, and in this particulate molten state at very high velocity propelled upon the target being coated. These processes, sometimes known as "metallizing", include Flame Spray (powder and wire), Plasma-Arc Spray (vacuum and atmospheric), Electric-Arc Spray, Detonation Spray and a recent technology development called High Velocity Oxygen Fuel (HVOF) spray. Metal and ceramic materials can be applied or 37 sprayed" from rod or wire stock and from powdered material. In the form of wire or rod, material is fed into the flame axially from the rear, where it is melted. The molten material is stripped from the end of the wire or rod and atomized by a high velocity stream of compressed air or another gas which then propels the material onto a prepared substrate or workpiece. In the electric-arc process two wires are electrically charged by a D.C. power supply. The wires are then feed into electrode tubes where arcing occurs between the wires. The heat of the arc produces molten metal that is then atomized by a compressed air stream and propelled onto a substrate to form a coating. The electric-arc process can only be used with electrically conductive materials. In powder form the material is metered, by a powder feeder or hopper, into a compressed air or gas stream which suspends and delivers the material to the flame. In the flame it is heated to a molten or semi-molten state then propelled to the work piece, where upon impact a bond is produced.
As molten or semi-molten particles impinge upon the substrate, one or more of several possible bonding mechanisms allow a coating to be built up. Mechanical bonding occurs when the particles "Splat" on the substrate and interlock with a roughened surface and/or other deposited particles forming a coating. With some combinations of substrates and coating materials localized micro-welding and/or diffusion alloying can occur. With some thermal spray coating systems, some bonding may also occur by means of Van der Waals forces. Analogous to this bonding would be the mutual attraction and cohesion which occurs between any two clean surfaces in intimate contact, e.g., the reflective coatings on mirrors, two optical flats or two gage blocks. Dependent upon the particular thermal spray coating process, coating material and substrate composition, any or all of these bonding mechanisms may come into play. However, for some applications and especially for thermal spray metallic coatings on metal targets or underlying metallic substrates, a bonding mechanism of metallurgical bonding is desirable. A metallurgical bond can be defined as adherence of a coating to the base material characterized by diffusion, alloying, or intermolecular or intergranular attraction at the interface between the sprayed particles and the base or other underlying material and usually is a stronger bond than a mechanical bond.
Among the thermal spray coating systems there are two, namely Vacuum Plasma Spray and Flame Sprayed and Fused processes providing products, which apparently can exhibit metallurgical bonding throughout at the interface of the thermally sprayed coating and its underlying base or substrate.
Vacuum plasma spraying (VPS) of high technology coatings is widely accepted throughout the world as a viable means for applying metallurgically bonded coatings. This process has proven to be an economical means for depositing most metallic and MCrAlY (multiple element alloys) coating materials used in the gas turbine industry. The high integrity coating produced by this process are usually pore free and metallurgically bonded.
Vacuum plasma spraying in inert atmosphere offers several unique advantages over conventional plasma spraying in inert atmosphere at atmosphere pressure.
To deposit a coating with optimum physical properties the spray material must maintain its original composition and metallurgical structure. These conditions are rarely achieved when depositing coatings in atmosphere conditions. In vacuum plasma spraying, the bond strength is increased because of higher substrate temperatures usually about 1600.degree. F., allowing the coating to partiality diffuse into the base material.
Spray deposition efficiency of the powder feed material can be increased because of increased particle dwell time in the longer heating zone of the VPS plasma. The coating produced by VPS are subjected to minimal changes in chemistry and metallurgy due to the chambers inert atmosphere.
The use of a plasma transfer arc process in vacuum is essential for achieving a metallurgical bond of the coating to the substrate. The plasma stream is electrically conductive, a secondary or transfer arc can be generated from the gun to the substrate provided the substrate is conductive. The substrate is negatively charged by a secondary D.C. power supply (approximately 300 amperes), this allows the energy of the arc to remove or sputter clean the substrate. This cleaning action creates a metallurgically clean surface and promotes bonding of the coating. A process of this type is described in U.S. Pat. No. 4,328,257. Post coating diffusion bonding of the VPS coating is normally accomplished in a vacuum furnace at 1950.degree. F. to 2050.degree. F. This heat treat operation completes the metallurgical bonding of the coating.
Normal operating procedures for VPS require the spray chamber be pumped down to approximately 400 .mu.m of Hg and then backfilled with inert gas (Argon) to 300 torr. Once the system has been sufficiently purged to achieve an acceptable inert atmosphere, the plasma spray operation is activated and the chamber pressure adjusted to the desired level for spraying. The entire spray operation is accomplished in a soft vacuum (approximately 50 torr). It should also be noted that the optimum spraying conditions will vary with the chemistry and particle size of each spray material. These variables are similar to conventional plasma spraying. Due to the complexity of low pressure spraying the entire process is best controlled by a computer, assuring complete reproducibility and homogeneity throughout the coating cycle.
Metallurgical bonding of thermally sprayed coatings also is achievable by a process called Flame Spray and Fuse. This process is a modification of the powder-flame spray method. The materials used for the coating are self-fluxing, fusible materials which require post-spray heat treatment. In general, these materials are nickel or cobalt base alloys which employ boron, phosphorous, or silicon (singly or in combination) as melt-point depressants and fluxing agents. In practice, parts are prepared and coated as in other thermal spray processes. Fusing is accomplished using one of several techniques; flame or torch, induction, or in vacuum, inert or hydrogen furnaces. These alloys generally fuse between 1850.degree. and 2150.degree. F. depending on composition. Reducing atmosphere flames should be used to insure a clean, well bonded coating.
In vacuum and hydrogen furnaces the coating may have a tendency to "wick" or run onto adjacent areas. Several paint-on stop-off materials are commercially available to confine the coating. It is recommended that test parts be fused, whenever the geometry, coating alloy, or lot of material is changed, to establish the minimum and maximum fusing temperatures. (The fusing temperature is known to vary slightly from lot-to-lot of spray material.) On vertical surfaces coating material may sag or run off if the fusing temperature is exceeded by more than a few degrees. Excessive porosity and non-uniform bonding are usually indicative of insufficient heating. Spray and fuse coatings are widely used in applications where excessive wear is a problem. These alloys generally exhibit good resistance to wear and have been successfully used in the oil industry for sucker rod, in agriculture for plowshares, etc. In most applications fusible alloys make possible the use of less expensive substrate materials. Coating hardness can be as high as R.sub.c 65. Some powder manufacturers offer these alloys with tungsten carbide or chrome carbide particles blended to increase resistance to wear from abrasion, fretting, and erosion. As mentioned earlier, these coatings are fully dense and exhibit metallurgical bonds. Grinding is recommended for finishing fused coatings because of the inherent high hardness. Use of spray and fuse coatings is limited to substrate materials which can tolerate the 1850.degree. to 2150.degree. F. of fusing temperatures. Fusing temperatures may also alter the heat treatment of some alloys. However, the coating will usually withstand reheat treating the substrate.
Thermal Spray devices used for most atmospheric coating applications can be hand held or machine mounted. Specially designed guns are commonly mounted on lathe compounds to spray cylindrical parts. Large flat parts are usually sprayed with guns mounted to two axis positioners such as those used by the welding industry. Complex parts requiring three or more axes of freedom can now be coated using commercially available, multiple-axis robots, and automated computer controlled systems. Using these techniques, geometries ranging from simple cylinders to complex air foils are being coated.
Thermal Spray coating is an effective, efficient means for altering surface characteristics of most materials. Thermally sprayed coatings enhance wear resistance, provide thermal barriers, and prevent hot corrosion/erosion of critical assemblies. The technology is essential to the aircraft engine and stationary gas turbine engine industry and is finding increasing applications in automotive, marine and industrial markets. There are many variables involved when producing thermally sprayed coatings, e.g., coating feed material, material flow rate, heat source control, substrate material and condition, and surface finish, etc. Coatings produced by this process are utilized throughout the world in almost every industry. Currently, the thermal spray process is widely used by all aircraft engine manufacturers for improving performance of civilian and military aircraft turbine engines. The aircraft repair and overhaul industry also utilizes thermal spray coatings for a variety of restoration and upgrade applications.
For additional background information on thermal spray coatings, reference is made to Metals Handbook, Vol. 5, Surface Cleaning, Finishing and Coating, 9th ed., American Society for Metals, Metals Park, Ohio, (1982) and particularly therein pages 361-374, "Thermal Spray Coatings", co-authored by J. H. Clare and D. E. Crawmer, with as much of pages 361-374 as necessary to complete this application's disclosure incorporated herein by this reference thereto.
As mentioned earlier, the condition of the substrate onto which the thermal sprayed coating is deposited is of great importance. Substrate surface cleanliness is of great importance in all thermal spray processes in order to ensure good bonding. As apparent from the just-mentioned Metals Handbook article entitled "Thermal Spray Coatings" and the portion of the article, pages 366-368, conventional surface preparation of the substrate surface is not taught to involve electrochemical cleaning thereof prior to thermal spray coating.
There exists in the coating technology a process called Selective Plating and also referred to as electrochemical metallizing. This electrochemical coating process, for example is described in an article entitled "Selective Plating", co-authored by D. W. Maitland and M. J. Deitsch, Metals Handbook, Vol 5, Surface Cleaning, Finishing and Coating, 9th ed., American Society for Metals, Metals Park, Ohio, 1982, pp. 292-299, with as much of this article as necessary to complete this application's disclosure incorporated herein by this reference thereto. Near the top of page 296 are taught the use of preparatory solutions to remove surface contaminants prior to selective plating. Briefly selective plating (i.e. electrochemical metallizing) is a molecular process in which the metal or alloy is being deposited molecule by molecule from a concentrated electrolyte bonding solution without using an immersion tank. The plating or bonding solution is in an absorbent material covering a portable anode or stylus which is connected to a special direct current power pack having the cathode lead of the power pack connected to the workpiece (i.e. the metallic surface to be coated). The stylus is moved in relation to the workpiece with the bonding solution there between and at the requisite voltage and current metal is deposited from the plating solution by contact of the solution-saturated anode with an area of the workpiece. In some ways selective plating is a process similar to a combination of arc welding and electroplating. The phenomenon involved creates a high level of adhesion to a workpiece surface which has been properly cleaned and activated. Because of high current levels employed, the metallic deposits are very dense, generally without voids and pore sites.